1. Field of the Invention
The present invention relates to a space vehicle thermal rejection system and more particularly to a space radiator utilizing separate heat pipe means.
2. Description of the Prior Art
Space vehicles are required to reject excess thermal energy to space. This requirement will continue to grow as future space systems get larger and have greater power. Space radiator systems are used in space thermal management systems for the functions of heat collection, heat transport, and heat rejection. The heat collection function includes interfacing with heat sources and transferring heat to the heat transport function. The heat transport function involves transmission of waste heat from the collection points to the rejection points. The heat rejection function uses space radiators for rejection of waste heat. Heat pipes, which are relatively simple devices that transport thermal energy by the evaporation, condensation, and return capillary flow of a two-phase working fluid, are useful in all three functions. This is because they provide high thermal conductivity in a self-contained, self-operating device without the need for valves, pumps, or compressors.
In a heat pipe, a working fluid circulates between heated and cooled regions to provide high thermal conductivity. In the heated region, called the "evaporator" section, thermal energy is transferred to the working fluid causing it to experience a phase change and become a vapor. This vapor then flows to the cooled region, called the "condenser" section, and becomes a liquid thereby releasing energy. The liquid is returned to the evaporator section by capillary action through a wick structure.
A planned United States Space Station will have high heat rejection loads. Two radiator concepts studied in the past to handle these loads are (1) the dual channel heat pipe attached to high efficiency fins, and (2) the honeycomb heat pipe. An example of the former is disclosed in U.S. Pat. No. 4,583,587 by Alario et al. It shows a monogroove heat pipe with a compact evaporator section, a condenser section encased in a heat radiating fin, separate channels for the axial transport of the liquid and vapor phases of the working medium, and a manifold that connects the evaporation section with the condensing section. Dual channel heat pipes generally have high heat transport capacity.
The honeycomb panel heat pipe concept has been studied as a space radiator. The concept has been described in AIAA paper 83-1430 (June 1983) entitled "High Capacity Honeycomb Panel Heat Pipes for Space Radiators". A honeycomb sandwich panel is used consisting of a wickable honeycomb core, internally wickable facesheets, and a working fluid. Evaporation of the working fluid occurs at any section of the panel exposed to heating. Vapor flows to a cooler region where it condenses, with the condensate returning to the evaporator by the capillary pumping action of the wick structure. Heat transfer is either from face-to-face, or in-plane along the faces.
The honeycomb panel heat pipe has good heat rejection performance, but limited heat transport capacity. The heat transport capacity can be improved by adding an external liquid sideflow, which is an external channel connected to the main vapor space via crossover connections. The sideflow offers low resistance to liquid flow in the direction of heat transfer, thus increasing the thermal transport that can be sustained by the pumping capillary forces. However, the increased heat transport capacity still does not match that of the dual channel heat pipe. Therefore, one way to optimize performance factors of a space radiator is to couple a high transport dual channel heat pipe, such as the monogroove, to a low transport radiating fin, such as the honeycomb heat-pipe panel. One disadvantage of doing this is the relatively high thermal resistance between the interfaces.